Plugins: Add validation stage to plugin loader pipeline (#73053)

* first pass

* change validation signature

* err tracking

* fix

* undo golden

* 1 more

* fix

* adjust doc

* add test helper

* fix linter

pkg/plugins/manager/pipeline/initialization/doc.go

@@ -1,6 +1,5 @@
 // Package initialization defines the Initialization stage of the plugin loader pipeline.
 //
 // The Initialization stage must implement the Initializer interface.
-// - Initialize(ctx context.Context, ps []*plugins.Plugin) ([]*plugins.Plugin, error)
 
 package initialization